This invention relates generally to programmable read-only memory (PROM) circuitry and, more particularly, to PROM circuitry having relatively large memory capacity.
As is known in the art, PROM circuits have a wide range of applications in digital computation and processing systems. As is also known in the art, such PROM circuits are typically formed as a single semiconductor integrated circuit chip. In bipolar PROM circuits, a matrix of rows and columns of conductors have memory elements connected between a unique row conductor-column conductor combination. Each one of the memory elements typically includes a diode and serially connected fusible link. During programming, selected ones of the fusible links are blown, creating an open circuit between the previously electrically connected row conductor and column conductor connected to such blown fusible link. The resulting pattern of blown and unblown fusible links represents data stored in the PROM. More particularly, a blown fusible link at a "location" defined by the unique row conductor-column conductor previously connected to such blown fusible link may represent a logical 0 signal stored at such location; whereas an unblown fusible link at a second address defined by a different row conductor-column conductor combination may represent a logical 1 signal stored at such second address.
More specifically, in the bipolar PROM circuit referred to above, each one of the row conductors is coupled to a corresponding row driver circiut. Each one of such row driver circuits includes an output transistor having a collector electrode connected to the row conductor coupled to such row driver circuit and an emitter electrode connected to a fixed potential, typically ground. When it is desired to fuse one of the fusible links coupled to a particular row conductor, base current is supplied to the one of the output transistors having its collector electrode connected to it to drive such transistor into saturation. Current is supplied to the one of the column conductors connected to the fusible link desired to be blown. Such current then flows through the selected fusible link, through the collector-emitter electrodes of the saturated output transistor to ground. Typically, the amount of current required to blow the fusible link is in the order of 25 milliamperes (ma). It is noted that, during the selection of one of the fusible links, the fusible links connected to the unselected row conductors have the diodes serially connected thereto reverse biased by a relatively high voltage produced at the collector electrodes of the output transistors of the unselected row driver circuits. While such voltage reverse biases these diodes, leakage current (typically 1.5 microampere per diode) does pass through such diodes and through the selected, saturated output transistor to ground potential. While in PROM circuits having a relatively small number of memory elements, typically less than 8 K, such leakage current is insignificant, in PROM circuits having, say, 16 K memory elements, the total leakage current passing through the saturated output transistor is in the same order of magnitude as the current needed to fuse or blow the selected fusible link. Therefore, the amount of current which such output transistor must sink is significantly increased as the memory capacity of the PROM is increased. One way to provide an output transistor which is able to sink this additional current is to increase the area required to form such transistor; however, such approach has the concommitant effect of reducing the amount of available area on the surface of the semiconductor chip.